Prerendering ignores webpages if a +server file exists in the same route, regardless of who exports prerender = true

[aenriii]

Describe the bug

SvelteKit can silently fail and/or refuse to prerender certain routes. This occurs when there is a +server.ts file in a route, regardless of its contents, next to a +page.svelte and a +page.server.ts with export const prerender = true. There are no warnings or other indications that prerendering has skipped this route, and SvelteKit informs the adapter that the route has been prerendered, leading to a 404 after build (see bun run preview).

Reproduction

clone https://github.com/aenriii/minrepro-svelte-prerender-fail with submodules, then

make deps
make test

Logs

System Info

System:
    OS: Linux 6.19 CachyOS Linux
    CPU: (16) x64 AMD Ryzen 7 2700 Eight-Core Processor
    Memory: 9.75 GB / 23.39 GB
    Container: Yes
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 22.22.2 - /usr/bin/node
    npm: 11.12.0 - /usr/bin/npm
    pnpm: 10.33.0 - /home/aenri/.bun/bin/pnpm
    bun: 1.3.4 - /home/aenri/.bun/bin/bun
    Deno: 2.7.8 - /usr/bin/deno
  Browsers:
    Firefox: 149.0
    Firefox Developer Edition: 149.0
  npmPackages:
    @sveltejs/adapter-auto: ^7.0.0 => 7.0.1
    @sveltejs/vite-plugin-svelte: ^6.2.4 => 6.2.4
    svelte: ^5.54.0 => 5.55.1
    vite: ^7.3.1 => 7.3.1

Severity

annoyance

Additional Information

Tested from main branch, commit checked added as a submodule to the reproducing repository

[aenriii]

been looking into it & i have a fix in progress;

the issue is in prerender.js, when the prerenderer visits a url it makes a plain GET with no Accept header, so the server's content negotiation sends it to the +server endpoint regardless of which file has prerender = true

the fix is simple,

• store page prerender and api prerender separately in metadata, then enqueue prerenders based on that
• add an expects_html param on visit() and enqueue() which adds Accept: text/html to the request

the only possible regression in my fix occurs if someone has a link on one page that both is prerendered and an api endpoint, as visit() is set to send found links in as implicitly expect_html = true. i'll work on a test for this, and see if it does happen and if so what can be done about it.

pr coming soon 🫡

[hyunbinseo]

Could this be a duplicate of #11794 or #10735 ?

[aenriii]

both of these seem to stem from the same root issue i was having but are from different perspectives;

the root issue that's happening is that when either the +page or the +server files call for prerendering, the prerenderer sends a request through sveltekit's internal router with no Accept header, which means that it will always go to the +server file regardless of whether or not thats the prerenderable file for two reasons;

1. sveltekit's content negotiation (wrt deciding between +page and +server) seems to be based on the Accept header signaling a preference for text/html
2. the visit() function in prerender.js doesnt get enough information from analyse.js->analyse() to know whether or not to send the proper Accept: text/html header.

this is only a duplicate issue in the root cause, but its two people experiencing 405s from a +server w/o a GET while mine was silently ignoring the webpage because the +server did have a GET.

my pr #15621 does still fix this issue, and past some ts definitions that i should be able to get past very easily it has no regressions i could find (and pnpm test in the prerendering folders fully passed, even with my regression tests)